Linus Pauling, ca. 1930s“In recent years I have formed, on the basis of mainly empirical arguments, a conception of the nature of the interatomic forces in metals which has some novel features.”
-Linus Pauling, 1938
Prior to the publication of this article, which appeared in the December 1938 issue of Physical Review, much about the interatomic forces operating in metals was either unknown, or theoretical predictions did not align properly with observed data. In publishing this paper, Linus Pauling first sought to align the incongruencies between theory and data for the transition metals, such as iron, cobalt, nickel, copper, palladium, and platinum. He was then able to correctly predict properties including “interatomic distance, characteristic temperature, hardness, compressibility, and coefficient of thermal expansion” by discarding previously held assumptions and inserting new – and correct – assumptions about transitions metals.
The most significant idea that Pauling introduced with this paper was the notion that the valence shell electrons – those in the outer shell – play a part in bonding. Previously, scientists believed that these electrons made “no significant contribution” to bond formation. Pauling was able to establish otherwise, and used this breakthrough to both align observable data with theoretical data, and make other predictions about transition metals.
The 1938 paper was written in the wake of a revolution within the world of chemistry. A raft of new theories brought about by a widening understanding of quantum mechanics was generating intense excitement for scientists world-wide, and the tools that quantum mechanics provided for helping to “correct” previous understandings of the chemical bond were of paramount interest to many. Pauling, of course, was a leader in this area, his body of work ultimately garnering the 1954 Nobel Chemistry Prize for “research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances.”
Within this area of focus, many scientists were especially interested in exploring the ways that metals bonded because, as noted, the observed data did not match up with theory. Pauling sought to mend this gap by using quantum mechanics to look at interatomic forces in a novel way. Prior to the Physical Review paper, chemists believed that when metals bonded, their valance shell electrons played only a small role in the resulting structures. Pauling argued otherwise, and put forth an important new theory that the valence shell electrons contributed to the process through resonance, a theory that he had developed earlier in the decade and continued to champion.
Because the crux of Pauling’s scientific intervention was to prove that valence shell electrons are involved in bonding, most of the paper is devoted to supporting this claim. The primary tool that Pauling uses to craft his argument is an analysis of temperature predictions. According to the reigning theory regarding metals and valence shell bonds, when bonding occurred, the electrons would bond in a manner that would create a moment of ferromagnetism. Specifically, it was theorized that these ferromagnetic moments would be temperature dependent, meaning that as the temperature of the metal changed, its degree of magnetism would also change in a predictable way. Experiments had shown however, that when metals bonded, their ferromagnetism remained independent of temperature.
Pauling exploited this piece of information and used it to support his theory. According to Pauling, if metals bonded through resonance, they would create ferromagnetic moments that were temperature independent, a hypothesis that correctly aligned with the observed data.
To develop his argument, Pauling made specific use of the element Vanadium, which has an electron configuration of 3d34s2. Under the old model, Vanadium’s valence electrons could only interact weakly in bonding if, at most, two of the 4s2 electrons were involved in a bond. This, according to Pauling, would create ferromagnetism which would decrease with increasing temperature, meaning that it was temperature dependent. On the contrary, the experimental evidence showed that Vanadium’s magnetism was temperature independent. This meant, therefore, that weak valence interaction during bonding was not possible.
Pauling’s alternative suggestion was that all of Vanadium’s valence electrons were involved in bonding through resonance; not just the two 4s2 electrons, as previously believed. Further, if the valence electrons bonded through resonance, the ferromagnetism of their structure would be temperature independent, a prediction that aligned with the observed data.
Once Pauling was able to prove that the valence electrons in Vanadium bonded through resonance, he then began to apply the concept to all transition metals. As with the previous example, Pauling continued to support the concept by comparing predicted outcomes with empirical data. And once again, when viewing the bonding through the prism of resonance, predicted outcomes of magnetic moments began to align with the empirical data.
Pauling then took it another step by repeating the exercise with interatomic distances. As demonstrated in his paper, a resonant structure would correctly predict the interatomic distances that had been observed for many bonds. Pauling also claimed that other properties, such as the “compressibility, coefficient of thermal expansion, characteristic temperature, melting point, and hardness” would likewise correctly align with experimental evidence, once resonance was used to explain valence shell bonding.
Though clearly a significant breakthrough, the assertions that Pauling made in his paper were grounded in work done by others — notably the quantum mechanical theory of ferromagnetism developed by Heisenberg, Frenkel, Bloch, Slater, et al., and Wolfgang Pauli’s theory of the temperature-independent paramagnetism of the alkali metals. And while the 1938 article acknowledges these debts, it also attempts to improve upon them.
This was especially so with the quantum mechanical theory of ferromagnetism. As we have seen, Pauling successfully applied the idea of temperature independence and ferromagnetism to support his claims, but he also found one aspect of the theory to be needlessly bothersome. As Pauling noted, in order for much of the theory to work on a mathematical level, scientists were compelled to assign positive numbers to all unpaired valence electrons. Pauling recognized that this was only necessary if it was assumed that valence electrons did not play a large role in bonding for metals. Under a resonant scenario, Pauling was able to show that the math could still work if the valence electrons were negative and that, once again, “this conclusion agrees with the observation.”
